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Journal of Water Process Engineering 9 (2016) 208–214

Contents lists available at ScienceDirect

Journal of Water Process Engineering

journa l homepage: www.e lsev ier .com/ locate / jwpe

emoval of fine iron-oxide particles after post-filtration in localotable water using an electrophoretic method

.Nazli Naim a,∗, Yeon Hui Ting a, Rabitah Zakaria a, Azhari Samsu Baharuddin a,hairul Faezah Md. Yunos a, Noor Fitrah Abu Bakar b,c, I.Wuled Lenggoro d

Faculty of Engineering, Department of Process and Food Engineering,Universiti Putra Malaysia, Selangor 43400, Serdang, MalaysiaFaculty of Chemical Engineering, Universiti Teknologi MARA, 40450, Shah Alam, MalaysiaCoRe of Frontier Material and Industrial Applications, Universiti Teknologi MARA, 40450, Shah Alam, MalaysiaGraduate School of Bio-Applications and Systems Engineering and Department Chemical of Engineering, 2-24-16 Nakacho, Koganei, Tokyo 184-8588,

apan

r t i c l e i n f o

rticle history:eceived 3 August 2015eceived in revised form 4 January 2016ccepted 10 January 2016

eywords:lectrophoretic depositionibre electrodesimiting fluxron oxideeta potential

a b s t r a c t

Potable water from several residential areas on the east coast of Malaysia was filtered using a polyether-sulphone (PES) membrane to separate the coarse and fine iron-oxide particles inside the pipelines. Theas-received samples consisted of a wide distribution of particle sizes, ranging from 5 �m to 400 nm. Theconcentration of fine iron-oxide particles inside a distribution system was extremely low. Hence, a spe-cific method is necessary to concentrate and separate the fine particles from the coarse ones. To studythe fine particles from the bulk, excess pressure was applied to the membrane filter so that the cloggedparticles were released into the permeate. A 100 kDa PES membrane was used to separate the parti-cles, because the samples consisted of a wide molecular-weight cut-off range from 89 g/mol goethite(�-FeOOH) to 231 g/mol hematite (Fe2O3). After the filtration process, the size distribution of perme-ated particles reduced to 550–400 nm. Through X-ray diffraction analysis, numerous polymorphs suchas �-FeOOH, Fe3O4, Fe2O3 and maghemite were detected from the samples. The zeta potential value ofthe permeated particles changed from −18.5 to −13 mV, suggesting that the dispersity of permeated

iron-oxide particles became unstable, but remained adequate for electrophoretic deposition (EPD). Thefibrous carbon electrode used in the EPD process, could remove up to 87% of the permeated iron-oxideparticles compared to solid carbon electrodes (<56%). A high-surface-area, porous electrode and a moder-ate applied voltage were preferred in order to minimise gas formation, reduce the electro-osmosis effectand increase the deposition efficiency.

© 2016 Elsevier Ltd. All rights reserved.

. Introduction

Drinking water, or potable water, is defined as treated waterhat is used for domestic purposes such as drinking, cooking andersonal hygiene. It is safe for consumption by humans with lowisk of immediate or long-term harm. In tropical regions such as

alaysia, potable water is distributed using pipelines that are maderom iron-based materials. During the distribution process, dete-ioration of the inner pipe interface occurs, owing to corrosion,

ontributing to the accumulation of iron-oxide particles in the bulk.t the initial stage of the corrosion process, the solid iron oxidesxist in low-crystalline form, but over time, the compounds trans-

∗ Corresponding author.E-mail address: mohdnazli@upm.edu.my (M.Nazli Naim).

ttp://dx.doi.org/10.1016/j.jwpe.2016.01.006214-7144/© 2016 Elsevier Ltd. All rights reserved.

form into higher crystalline forms, such as goethite (�-FeOOH) orhematite (Fe2O3). Dixit and Hering pointed out that the rate of thesetransformations depends on temperature, pH and the presence ofother co-occurring solutes [1]. During the corrosion process, themorphology of the inner iron pipe surfaces consists of a porouscore, a hard shell layer (HSL) and a surface layer. The porous coreregions consist of enormous fine tubercles that are composed of�-FeOOH, lepidcrocite (�-FeOOH), green rusts, magnetite (Fe3O4),Fe2O3, ferrous hydroxide [Fe(OH)2], ferric hydroxide [Fe(OH)3] andsiderite (FeCO3). The HSL lies on top of the core region and variesfrom one to a few millimeters in thickness [2], and it is typicallycomposed of Fe3O4 and �-FeOOH [3,4]. Generally, the outermostor surface layer can be up to several millimeters thick. The layer is

more heterogeneous and can contain �-FeOOH, �-FeOOH, Fe(OH)3,silicates, phosphates and carbonates [5]. Sarin et al. and Senftle et al.explained that, under ambient conditions, the corrosion process is

er Process Engineering 9 (2016) 208–214 209

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Table 1Analysis of the as-received samples according to the Malaysian Food Act, Water andPackaged Drinking Water Standard (Health) [28].

Parameter Results, g/m3 Standard, g/m3

Colour <2.5 15Turbidity 1.5 5pH 6.5 6.5–8.5Aluminium, mg/L Not detected (<0.06) 0.2Arsenic, mg/L Not detected (<0.0001) 0.05Cadmium, mg/L Not detected (<0.003) 0.005Chromium, mg/L Not detected (<0.03) 0.05Copper, mg/L 0.05 1.0Cyanide, mg/L Not detected (<0.003) 0.1Fluoride, mg/L Not detected (<0.01) 1.5Total hardness, mg/L 1.36 500Iron, mg/L 0.05 0.3Lead, mg/L 0.01 0.05Magnesium, mg/L 0.16 150Manganese, mg/L Not detected (<0.002) 0.001Mercury, mg/L Not detected (<0.0001) 0.001Mineral oil, mg/L Not detected (<0.1) 0.3Nitrate, mg/L 0.16 10Phenol, mg/L Not detected (<0.001) 0.002Residual Chlorine, mg/L Not detected (<0.1) 0.1Selenium, mg/L Not detected (<0.0001) 0.01Silver, mg/L Not detected (<0. 005) 0.05Sodium, mg/L 0.82 200

M.Nazli Naim et al. / Journal of Wat

ominated by iron ionisation followed by solid reduction before theron is released as a solid ferrous form [4,6]. Later, it could eithere dispersed in the bulk or deposited as a scale on the pipe-wallurface [7,16]. The solid fragment of iron-oxide substances or tuber-les that are released into the potable water initially exist in theilimeter-to-submicron size order. The coarse tubercles are easy

o sediment, but the fine tubercles are well-dispersed and can flowomogeneously inside pipelines or distribution systems. Fine iron-xide particles that exist in the nanometer size order have a higherurface activity compared to coarse ones, and have the potential toorm natural adsorbent particles. As a result, the fine tubercles thatre exposed with a long residence time inside a distribution sys-em will turn into coarse particles with the potential of promotingiofilm growth, microbial algae activities [3,8], green rust [9] anddsorption or accumulation of some contaminant such as arsenic10] and radium [11].

The concentration of fine iron-oxide particles inside a distri-ution system is extremely low and difficult to study. In order totudy the particles, a specific method is necessary to concentratend separate the fine particles from the coarse ones. In the presenttudy, a batch filtration process is used to increase the concen-ration of iron-oxide particles. The collected iron-oxide particlesere filtered and trapped inside a membrane matrix. By increasing

he applied pressure to slightly more than the limiting flux value,he fine iron-oxide particles that were trapped in the matrix wereeleased to the permeate and could be measured and studied. Theeleased iron-oxide particles were subjected to a removal processy means of electrophoretic deposition. According to Naim et al.,lectric-assisted separation or electrophoretic deposition (EPD)as conducted, because of its potential to be applied directly, with-

ut any additional treatment or chemicals [12]. However, beforePD could be applied, a preliminary study on the characterisa-ion of permeated iron-oxide particles that were released to theermeate was necessary. This study also emphasised the poten-ial of removing and depositing the permeated iron-oxide particlesfter applying the EPD system in terms of removal efficiency andeposit-substrate morphology. Characterisation of fine iron-oxidearticles in the pre- and post-filtration steps is important in order totudy the feasibility of applying EPD in the permeate region or afterhe filtration process. In this study, a particular concern was giveno the permeation of iron-oxide particles and the surface chargef the iron-oxide particles after passing the membrane. The EPDrocess is preferable if the permeated iron-oxide particles have aigh surface charge or adequate zeta potential value. If the sur-

ace charge is completely stripped off by the membrane matrix,n additional treatment might necessary to increase the surfaceharge of the particles. Particular concerns when studying the sep-ration parameters, such as limiting flux and applied pressure, areaken into account so that losses of particle charges through con-entration polarisation and friction within the membrane matrixan be minimised. At the end of this work, an alternative methodo increase the concentration of fine iron oxides removed by EPDas also demonstrated by increasing the electrode surface area and

orosity.

. Experimental

.1. Collection of samples

A volume of 10 L of potable water was taken from several resi-ences in Kuantan, Pahang, Malaysia. The tap water was collected

n plastic bottles and tightly closed to prevent interaction with the

urrounding environment. The taps and the pipelines were esti-ated to be about 10 years old and have a diameter of 2.5 cm.

oth the taps and the pipes were made from galvanised ironBS1387:1967). The collected water samples were tested according

Sulfur, mg/L Not detected (<0. 01) 400Zinc, mg/L 0.05 5

to the national standards for water quality (Malaysian standard), asshown in Table 1 [28].

2.2. Filtration unit and membrane selection

A membrane made of polyethersulphone (PES) with an activesurface area of 40 cm2 was used as a filter for the filtration process.The filtration unit and the membrane system were purchased fromSartorius Stedim Malaysia Sdn. Bhd, Malaysia. The membrane had a10 nm pore size with a 100 kDa molecular weight cut-off (WMCO).The process flow of membrane filtration from the source until theanalysis stage is shown in Fig. 1.

2.3. Concentration of iron-oxide particles

The concentration of iron-oxide particles for samples as-received, permeated and remaining after the deposition wasmeasured using ICP-OES (iCAP6000, Thermo Scientific, USA). Theremoval percentage calculation was performed to identify the EPDefficiency. The inductively coupled plasma was used to identify themetal element. The nitric acid digestion method was performed toreduce the particulates in order to form free ions.

2.4. X-ray diffraction (XRD) analysis

Each batch of the water samples was pre-filtered using a 0.2 �mpolytetrafluoroethylene (PTFE) syringe filter. The purpose of pre-filtration is to avoid uncontrolled clogging, owing to the very wideparticle size distribution. The process also removes unwanted con-taminants such as debris and coarse natural organic matter (NOM),so that membrane filter clog can be minimised [13]. The thin filmof membrane with accumulated particles was calcined in the fur-nace at 600 ◦C for 5 hours, and then preserved in the furnace forseveral hours in order to promote the crystallisation process [14].The calcinated powders were analysed using an X-ray diffractome-

ter (Model APD 2000, G.N.R., Italy). A copper anode was used as aradiation source (CuK�) with a wavelength of 1.54 nm. A voltageof 40 kV and current of 30 mA were used in XRD analysis [13]. TheXRD analysis was perfor ed with scanning angles between 2 and 80◦

210 M.Nazli Naim et al. / Journal of Water Process Engineering 9 (2016) 208–214

F alysesE

aga

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ig. 1. Schematic illustration of the experimental method and the conducted anlectrophoretic deposition (EPD) was conducted after the filtration process.

nd the speed was set at 2◦/sec with step scans at 0.05◦ [14]. Theenerated peaks were analysed to identify the types of substancend other polymorphs available in the samples.

.5. Membrane filtration

The membrane unit purchased from Sartorius (Vivacell 250,ermany) consists of sample reservoir, permeate container, PESembrane (100 kDa MWCO), pressure indicator and an inlet valve

or compressed-air supply. The membrane was inserted into theample reservoir with the volume marking facing the transparentlot. The water samples were kept inside the sample reservoir andapped with the pressure head. To observe the applied pressure, aauge was installed on the valve that was connected to compressed

ir. An oil-free air compressor was installed in order to avoid oil orebris contamination during the filtration process. The inlet pres-ure was applied from 0.5 to 3.5 bar. A beaker was placed under theembrane unit to collect any discharged permeate.

. Small SEM image at the top right indicates the collected iron-oxide tubercles.

2.6. Deposition and removal of iron-oxide particles

Carbon plates (25 × 5 mm2) were submerged into 50 mL of thecollected permeate water samples. The substrate distance anddeposition time were set at 30 mm and 10 min, respectively. Anelectric field was applied using a direct current (DC) during theEPD. During the EPD, three different values of supply voltage; 5,15 and 25 V, were applied to the samples. The distance betweenboth electrodes was fixed at 2 cm. Details about the experimen-tal setup are available from our previous work [12,30]. The sameEPD process was also applied, using fibrous carbon substrates asa comparison to the earlier method. The deposits that attached tothe substrates were dried for 5 h in a vacumm oven at 60 ◦C. Todetermine the iron-oxide removal, Eq. (1) was applied after the

EPD process [12,30].

Removal(%) = Ci − Cf

Ci× 100 (1)

M.Nazli Naim et al. / Journal of Water Process Engineering 9 (2016) 208–214 211

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Table 2Comparisons of the iron-oxide polymorphs in the collected water samples and theanalysed galvanised metal pipelines from Baek et al. [17].

Corroded substance In water samples In pipe wall surface [17]

Hematite, �-Fe2O3 Available Not availableGoethite,�-FeOOH Available AvailableMagnetite, Fe3O4 Available AvailableMaghemite, �-Fe2O3 Available AvailableSiderite, FeCO3 Not available Available

ig. 2. XRD analysis of dried solid content collected on the pre-filtered surfacecalcined at 600 ◦C, 5 h).

here Ci (g/m3) is the initial concentration and Cf (g/m3) is thenal concentration of iron (Fe) after the EPD process.

.7. Zeta-potential measurements

Each sample was measured with zeta-potential measurementpparatus (Nihon Rufuto Co. Ltd., Japan). During the sample mea-urement (assuming contaminant was consisted of diluted ironxide in aqueous), the refractive index was set at 3.22 with anbsorbance value of 1.0.

.8. Particle size measurement

The particle size of as-received and the permeated samples wereeasured using a Malvern ZetaSizer Nano-ZS (Malvern Instru-ents, UK). Each sample was measured three times before the

verage particle size for each sample could be determined. Owingo the soft aggregation between solid particles, NOM accumulationnd fouling of the particles on the wall container, the samples wereltra-sonicated at 53 kHz for 2 min to enhance particle dispersion.

. Results and discussion

.1. Water characteristics and collected solid content

The characteristics of the as-received samples are shown inable 1. The results show that all detected compounds inside theulk follow the standard, except for the iron (elemental) material.

t was noticed that the amount of iron slightly exceeded the stan-ard limit when the treated water samples were collected from austed distribution system.

From the XRD analysis, the filtered solid particles or retentatecollected from as-received samples) consist of several iron-oxideolymorphs, as detailed in Fig. 2. Five different peaks of iron-oxideolymorphs were identified after calcining the retentate at 600 ◦C

or 5 h. The retentate must be heated in order to remove the NOMhat screen the solid content. The iron-oxide polymorphs wereetected using XRD, whereby the major species identified were �-eOOH at 26.4◦, Fe3O4 at 35.5◦, maghemite at 42◦ as well as Fe2O3t 32.9◦ and 48.9◦ [15,16]. Fe2O3 and maghemite were detectedecause of the transformation of �-FeOOH and Fe3O4 when theusbtances were heated above 600 ◦C. Therefore, it was confirmedhat �-FeOOH and Fe3O4 were the initial forms of iron oxide sus-ended in the water sample, which was in good agreement withrevious works reported by Baek et al [17]. Legodi and De Waal

15]. Lin et al. [14] and Sarin et al. [4]. Table 2 shows the compar-sons between the iron-oxide polymorphs found in the samples andhe corroded iron-based pipe surfaces [17].

Fig. 3. Permeate flux pattern of the collected water samples with various appliedpressures using a PES membrane (all samples were sonicated at 53 kHz, 2 min beforethe filtration process). Internal fouling occurs after 120 s.

3.2. Filtration of iron-oxide particles

The filtration process was performed using a PES membranewith a MWCO of 100 kDa. As the iron particles in the water rangefrom 88.85 g/mol (�-FeOOH) to 231.55 g/mol (Fe3O4), penetra-tion of fine particles through the PES membrane is possible. Theexistence of fine iron-oxide particles inside the permeate was con-centrated if most of the trapped particles within the membranematrix were released. Fig. 3 shows the effect of PES membrane fil-tration at various applied pressures, ranging from 1.5 to 3.5 bar.The permeate flux decreased with time, owing to the retention ofsolutes or particulates on the outer layer of the membrane surfaceduring the prefilter stage. Initially, a high flux rate, 30 × 10−4 m/s,was observed when the applied pressure was higher than 1.5 bar.After 120 s, internal fouling was already dominant in the process,owing to the increase in the number of deposited particles on themembrane surface. As the applied pressure increased between 2and 3.5 bars, the fouling was more apparent, whereby the fluxexceeded the membrane limiting value, which promotes the con-centration polarisation and forms a gel layer [18]. Under theseconditions, the accumulated iron particles formed complex aggre-gates and were unable to pass through the membrane. Whenthe critical conditions were exceeded, the excess applied pressureforced the clogged solutes to penetrate through the membrane.Fig. 4 shows the conditions under which fouling occured and theflux reached the membrane limiting flux, Jlimit. After reaching thislimiting flux, a significant increase in particle size could be clearlyobserved in the permeate solution (see Fig. 5). The permeate fluxcomparison was performed for pure water and the collected watersample to observe the fouling effect. The pure water sample did notproduce any fouling or reach the membrane limiting flux, whichindicates that pure water is free from fine particulates.

Here, we assumed that the transport of iron-oxide particlesthrough the membrane pores is governed by particle motion onthe membrane system, which includes a combined effect of electro-static repulsion, Brownian forces and particle hydrodynamic drag.

In normal flow filtration, the large particles are transported awayfrom the membrane surface through lift forces, allowing one tooperate filtration devices at high filtration velocities without sig-

212 M.Nazli Naim et al. / Journal of Water Process Engineering 9 (2016) 208–214

Fig. 4. Permeate flux comparison between pure water and the collected water sam-ples under various applied pressures from 0 to 3.5 bar. Zero pressure represents theunfiltered, collected water.

Fww

ntdnptBppbcfteTdafv

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Fig. 6. Deposition of iron-oxide particles from a membrane filter on carbon-plate

ig. 5. Particle size and zeta potential values of the solid content in the collectedater samples and the permeate. Zero pressure represents the unfiltered, collectedater.

ificant fouling or polarisation. However, Kim and Zydney claimedhat small particles tend to deposit on the membrane surface, as therag force from filtration is greater than the inertial lift [19]. Brow-ian forces have a large effect on the particle trajectories, allowingarticles to enter the pore even under conditions at which the elec-rostatic repulsion is greater than the hydrodynamic drag force.elow the critical filtration velocity or membrane limiting flux, thearticles attain an equilibrium position above the membrane at theoint whereby the hydrodynamic drag forces are exactly balancedy electrostatic repulsion. At small values of the electrostatic forces,orresponding to very high filtration velocities, the particles simplyollow the fluid streamlines through the membrane. Thus, whenhe membrane limiting flux is exceeded, the fraction of particlesntering the pores increases with increasing filtration velocity [19].he theories were also supported by Richard et al., whereby theyefined the “critical filtration velocity” or membrane limiting fluxs the fluid velocity within the membrane pore at which the netorce acting on a particle, located at fixed position above the pore,anishes [20].

Fig. 5 shows the characterisation of particulates, that is, the. par-icle size and zeta potential values of the iron oxide in the permeate.he particle size and zeta potential of the iron-oxide particles inhe water before filtering were 450 nm and −18.5 mV, respectively.fter passing the water sample through a 100 kDa PES membranet 1.5 bar, nanometer-order iron-oxide particles appeared in theater. The particle size in the permeate was 340 nm, with a zeta

otential value of −13 mV. This suggested that the iron-oxide par-icles in the treated water were moderately stable. However, afterhe Jlimit was exceeded at an applied pressure of 2 bar, the particleize in the permeate increased from 340 to 434 nm and the zeta

and carbon-fibre electrodes. A high percentage removal can be obtained using thecarbon-fibre electrodes located on the cathode side.

potential values reduced from −15.14 to −11.78 mV. The aggre-gate size also increased from 434 to 700 nm with increasing appliedpressure. Starting from 2 bar onwards, it is assumed that most ofthe fine particles were coming from the clogged filter matrix. Inour hypothesis, aggregation of iron-oxide particle in the permeateoccurred because of interparticle collisions during the concentra-tion polarisation that promoted gel polarisation or because of thecompaction of iron-oxide particles inside the membrane matrix[21]. This hypothesis is consistent with the studies reported byHarmant and Aimar and Petsev et al. [18,22].

The aggregation of iron-oxide particles could also be attributedto multi-polar surface charge interactions that contribute to par-tial neutralisation inside the membrane matrix. The multi-polarsurface charge occurs because of the presence of adsorbed contam-inants such as natural organic matter (NOM) and other adsorbedmetal arsenic that shifts the zeta potential values [23,24]. In addi-tion, free ions such as Na+, Ca2+ and K+ that were present inthe water samples could also cause multi-polar surface charg-ing. The ions were subjected to a shielding effect in the electricaldouble layer of the iron-oxide particles (thinner electrical dou-ble layer) in the water, thus reducing the zeta potential value[25].

3.3. Deposition and removal percentage

Fig. 6 shows the removal of iron particles in water samplesusing two different types of substrates, a carbon plate and acti-vated carbon filter. The difference between these two substrates isthe deposition rate, owing to the substrate morphology and sur-face area. The carbon plate in this study is a rigid material, whereascarbon fibre is more flexible and exists in fibrous form. The carbonfilter offered a higher surface area compared to the carbon platesubstrate, owing to the fibre arrangement. The FESEM images ofthe carbon fibre before and after EPD are shown in Fig. 7. A lowapplied potential is preferable in this study, because a high levelof gas formation is observed at the high voltage. The amount ofdeposited particles increased as the applied potential increased,suggesting that the yield or deposits follow the EPD model, as pre-dicted by Biesheuvel et al. and in from our previous work [26,29,30].Owing to water electrolysis, particular concern should be given tothe bubbles produced in the vicinity of the substrates, because itcould affect the particle deposition uniformity [27,30]. Optimumremoval occurs at <25 V, where 80–87% removal was obtained usingthe carbon fibre, whereas 26–56% removal was achieved using the

carbon plates. The obtained particles with moderate zeta potentialvalues were stable in the suspension and could deposit during theEPD process.

M.Nazli Naim et al. / Journal of Water Process Engineering 9 (2016) 208–214 213

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ig. 7. FESEM images of the carbon-fibre electrodes a before and b after depositions indicated with round circles).

. Conclusion

Throughout this study, it is proven that drinking water from thepecific distribution system:

. Consists of the mentioned iron-oxide polymorphs hematite,magnetite and maghemite.

. Contains particles that tend to aggregate because of compactioninside the membrane filter matrix when the iron-oxide parti-cles pass through the filtration system with an applied pressurehigher than Jlimit.

. Consists of penetrated iron oxide in the permeate region withmoderate—low zeta potential values, which get lower as theyexceed Jlimit.

. Has a high percentage removal of up to 87% when using a high-surface-area substrate with a 2 cm electrode distance and 5–25 Vapplied potential.

. Contains iron particles with high zeta potential value that canbe efficiently removed using electrophoretic deposition method;however, those with a low zeta-potential value close to zero areunable to deposit.

cknowledgements

The authors are grateful to Dr. Rajagopalan Prabu for his techni-al contribution and Ms. Syahira Mohd. Sharif for the experimentaletup. This study was supported by Universiti Putra Malaysia,niversiti Teknologi MARA (UiTM) and Ministry of Education

alaysia through financial support of Universiti Putra Malaysiarant (GP-IPS/2013/9399815), UiTM Research Entity Initiative

REI) Tier5 Grant (600-RMI/DANA 5/3 REI (2/2013)) and Fundamen-al Research Grants Scheme (FRGS/2/2013/TK05/UPM/02/15).

[

nification of the fibre strands c before and d after the deposition (deposit particles

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